1. Field of the Invention
The present invention relates to an image forming apparatus such as a copier, a printer, a facsimile machine or a similar electrophotographic image forming apparatus. More particularly, the present invention relates to a developing device which uses one component developer utilized in such image forming apparatuses.
2. Discussion of the Background
A developing device which uses one component developer is favorable for miniaturization and to decrease cost. In order to avoid deformation of a developing roller, wearing of a surface of a developing roller and forming a ghost and to reduce torque, a non-contacting type developing device in which toner on a surface of a developing roller jumps across a distance between a surface of a developing roller and a surface of a photoconductive drum is favorable.
In a background one component and non-contacting type developing device, if a cohesive force of toner is high, an adhesive force between the toner and a surface of a developing roller and between toner particles is high. As a result, it is difficult to transfer toner from the surface of the developing roller to a surface of a photoconductive drum, and therefore image density becomes low and gradations of an intermediate density of the image become poor.
In order to improve liquidity of toner, Japanese-Publication No. 63-42787 discloses a one component developing device in which additives such as silica are added to the toner. Silicon carbide or titanium oxide are also usable as additives.
Further, in a background one component developing device, it is difficult to carry enough toner on a developing roller, and therefore image density becomes poor.
In order to solve such problems, Japanese Laid-Open Patent No. 4-127177 discloses a developing device in which small size electric conductive portions which are grounded and dielectric portions are provided on a surface of a developing roller. Toner and the dielectric portions are triboelectrically charged at a contact portion between a developing roller and a toner supply roller which moves relative to the developing roller. A large number of minute electric fields are formed on a surface of the developing roller by charging the dielectric portions, and therefore charged toner is adhered to the surface of the developing roller for carrying the developer, and a few toner layers are formed on the developing roller.
The present inventors have conducted experiments on a developing roller which was the same as the developing roller of the Japanese Laid-Open Patent No. 4-127177 and on developer which was the same as the developer of the Japanese Publication No. 63-42787. As a result of the experiments, the present inventors determined that a density of the toner became poor after a large number of image operations were executed. The cause of the problem is that since additives which increase the liquidity of the developer are harder than parents of toner (i.e., base toner particles) such as styrene-acrylic resin and polyester resin, the additives are buried in the parents of toner while the developer is mixed by an agitator in a developer container, and therefore toner which is not developed increases. Further, an adhesive force between the toner and a developing roller increases since the parents of toner are held in direct contact with the developing roller, and as a result the toner adheres to the developing roller. Especially, since the developing roller of the Japanese Laid-Open Patent No. 4-127177 has a strong adhesive force to toner, it is difficult to transfer toner to a photoconductive drum.
Japanese Publication No. 2-60179 discloses a developer in which resin particles having an average diameter from 0.1 .mu.m to 2 .mu.m are added to the developer from 0.01 to 10 weight-% to increase an ability of triboelectric charge of the toner. The ability of the triboelectric charge of the toner becomes poor if grinding particles such as silica are added to the developer to avoid toner filming from occurring on a surface of a photoconductive drum. According to the experiments by the present inventors, it was difficult to obtain enough liquidity and enough developing efficiency even if developer which includes resin particles was used in the one component and non-contact type developing device.